1. Technical Field
The present invention relates to a vacuum/reduced pressure refining process and a vacuum/reduced pressure refining facility for use in metal refining, i.e. refining of alloys such as steel, etc., e.g. molten metal, etc. in a vacuum/reduced pressure converter, a degassing apparatus for a vacuum ladle, etc.
2. Description of Related Art
In continuous operation of vacuum/reduced pressure refining treatment under a subatmospheric pressure, the following steps are repeatedly carried out in series to exchange the treated molten metal in the refining vessel with an untreated molten metal.
That is, in case of a vacuum/reduced pressure refining apparatus of such a model as shown in FIG. 8, at first molten metal 13 is charged into a refining vessel 1, followed by closing the vessel with a lid 14 and reducing the inside pressure of the refining vessel to vacuum/reduced pressure. In case of a vacuum/reduced pressure refining apparatus, as shown in FIG. 9, a ladle 17 containing the molten metal is placed in a refining vessel 1, followed by closing the vessel with a lid 14 and reducing the inside pressure of the refining vessel to vacuum/reduced pressure. In the case of a vacuum/reduced pressure refining apparatus, as shown in FIG. 10, a ladle 17 containing molten metal 13 is positioned on a ladle elevating device 18 under a refining vessel 1, followed by dipping the lower end 19 of the refining vessel 1 into the molten metal and reducing the inside pressure of the refining vessel to a vacuum/reduced pressure. After the vacuum/reduced pressure treatment, the inside pressure of the vacuum/reduced pressure refining vessel is returned to atmospheric pressure, followed by removing the lid from the refining vessel to open the vessel or by removing the lower end of the refining vessel from the molten metal in the ladle. Then, the treated molten metal is discharged from the refining vessel, or the ladle is taken out. The period from the end of these operations to the start of the next treatment is a waiting period.
Use of a filter-type dust collector in a vacuum evacuating apparatus is known, for example, from JP-A-617115. In such a system the dust collector must be connected to a vacuum/reduced pressure refining vessel and used in a hermetically closed state during the vacuum/reduced pressure refining treatment, and thus there is no suction of excess air therein during the treatment. When dust in an unoxidized metallic state is generated in the vacuum/reduced pressure refining vessel, the dust reaches the dust collector, while maintaining the unoxidized state. As a result, when air invades the dust collector for reasons such as pressure returning to the atmospheric pressure by air, etc., the metal dust deposited on the filter react with the air, thereby causing the problems of oxidation/heat generation. As a result, in the case of filter cloth being used for a filter, the filter cloth is damaged by heat or completely burnt in serious cases. In the case of ceramics being used for a filter, the filter itself undergoes no direct damage by heat, but the collected dust is sintered to cause clogging in the filter meshes or impair the filterability of the filter due to solidification on the filter.
To solve these problems, JP-A-8-3627 discloses that, in the case that combustible substances are contained in the dust, the dust collector must be subjected to pressure returning or back washing with an argon gas or a nitrogen gas to prevent the filter from being damaged by the air introduced at the time of pressure returning after the vacuum degassing treatment of treated molten metal.